biol102

Copyright © 2010 Pearson Education, Inc.

BIOL102


Origin of Species

Part 1 – A few reminders from lecture 2• Modern Synthesis of Genetics and Evolution• Hardy-Weinberg Principle• Factors Changing Allele Frequencies

BIOL102

Source of cover picture: Reece et al. (2010) , Campbell Biology, 9th edition, Pearson Benjamin Cummings, San Francisco (CA), Figure 24.4c


Part 2 – Species Concepts• Species• Biological Species Concept• Morphological Species Concept• Ecological Species Concept• Phylogenetic Species Concept

BIOL102

Origin of Species


Part 3 – Speciation• Allopatric Speciation• Sympatric Speciation• Rates of Speciation• Dynamics

BIOL102

Origin of Species


• A population is the smallest biological unit that can evolve and is defined as a group of individuals of the same species that live, interbreed and produce fertile offspring in a particular geographic area • A gene pool consists of all alleles (forms of genes) for all loci in a population and is the source of genetic variation that produces the phenotypes and their traits on which natural selection acts• A population evolves when individuals with different genotypes survive or reproduce at different rates

Modern Synthesis of Genetics and Evolution

Part 1 – A few reminders from lecture 2


• states that frequencies of alleles and genotypes in a population remain constant from generation to generation if certain conditions are met (Hardy-Weinberg equilibrium) no mutations random mating no natural selection extremely large population size (no effect of genetic drift) no gene flow (migration into or out of a population)


Hardy-Weinberg Principle


• Hardy-Weinberg equilibrium is a null hypothesis, which assumes that allele frequencies are not changed• However, there are at least four mechanisms of evolution, which cause changes in allele frequencies of populations: mutations gene flow genetic drift natural selection

Factors Changing Allele Frequencies



• is defined as an evolutionarily independent population or group of populations• Biologists commonly use the following four approaches to identify species: the biological species concept the morphological species concept the ecological species concept the phylogenetic species concept

A. Species

Part 2 – Species Concepts


Species


• defines a species as a population or group of populations whose members have the potential to interbreed and produce fertile offspring• considers populations to be evolutionarily independent if they are reproductively isolated from each other and no gene flow occurs between them

B. Biological Species Concept



• Biologists categorize the mechanisms that stop gene flow between populations into prezygotic barriers (before fertilization) and postzygotic barriers (after fertilization) prezygotic barriers: individuals of different species are prevented from mating postzygotic barriers: individuals from different populations do mate, but the hybrid offspring produced have low fitness and do not survive or produce offspring

Prezygotic and Postzygotic Barriers

Biological Species Concept


Prezygotic and Postzygotic Barriers

Individuals ofdifferent species

Mating attempt

Fertilization(zygote forms)

Viable, fertileoffspring

Prezygoticbarriers

Postzygoticbarriers


• block fertilization from occurring by: impeding different species from attempting to mate preventing the successful completion of mating hindering fertilization if mating is successful

Prezygotic Barriers



• Habitat isolation: two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers• Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes• Behavioral isolation: courtship rituals and other behaviors unique to a species are effective barriers

Prezygotic Barriers


• Floral traits of plants can influence the behavior of pollinators, and thus whether plants can hybridize two species of columbines (Aquilegia) in California can produce fertile hybrids, but flower structure determines that one species is pollinated by hummingbirds, the other by hawkmoths, so hybridization is rare

Behavioral Isolation


Behavioral Isolation


• Mechanical isolation: morphological differences (e. g., size and shape of reproductive organs) can prevent successful mating• Gametic isolation: sperm of one species may not be able to fertilize eggs of another species

Prezygotic Barriers


• In plants, mechanical isolation may involve pollinators many orchid flowers look and smell like the females of particular pollinator species male insects attempt to mate, thereby transferring pollen

Mechanical Isolation


Prezygotic barriersHabitat

IsolationTemporalIsolation

BehavioralIsolation

MechanicalIsolation

GameticIsolation

Individuals of

differentspecies

MATINGATTEMPT FERTILIZATION

(a) (c) (e) (f)

(b)

(g)

(d)

Prezygotic Barriers


• prevent the hybrid zygote from developing into a viable, fertile adult due to: reduced hybrid viability reduced hybrid fertility hybrid breakdown• Hybrids are the offspring of crosses between different species

Postzygotic Barriers



• Reduced hybrid viability: genes of the different parent species may interact and impair the hybrid’s development• Reduced hybrid fertility: even if hybrids are vigorous, they may be sterile• Hybrid breakdown: some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile



Reduced Hybrid Fertility


Reduced HybridViability

Reduced HybridFertility

HybridBreakdown

FERTILIZATIONVIABLE,FERTILE

OFFSPRING

Postzygotic barriers

(k)

(h) (i)

(j)

(l)



• If two formerly isolated populations are reunited before complete reproductive isolation has developed, interbreeding can occur with three possible outcomes: if hybrid offspring are as fit as those resulting from matings within each population, hybrids will mate with individuals of both parental species. The gene pools will gradually become completely mixed (no speciation)

Hybrid Zones



if hybrid offspring are less fit, reinforcement may result in more prezygotic barriers and complete reproductive isolation may evolve (speciation) a hybrid zone may develop in the absence of reinforcement, or before reinforcement is complete, and may contain recombinant individuals resulting from many generations of hybridization

Hybrid Zones


• Example: two species of European toads have a long narrow hybrid zone the toad hybrids have many defects, some of which are lethal on average, a hybrid toad is significantly less fit as a purebred individual the hybrid zone is narrow, because there is strong selection against hybrids. But it persists because individuals of both species continue to move into it and mate

Hybrid Zones


Hybrid Zones


Limitations


• The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually for example, prokaryotic and viral species must be defined differently• this concept can only be applied to populations that overlap geographically• it also emphasizes absence of gene flow, which can occur between distinct species for example, grizzly bears and polar bears can mate to produce “grolar bears”


Grizzly bear (U. arctos)

Polar bear (U. maritimus)

Hybrid “grolar bear”

Limitations of the Biological

Species Concept


• defines a species by differences in morphological or structural features is based on the idea that distinguishing features are most likely to arise if populations are independent and isolated from gene flow applies to sexual and asexual species but relies on subjective criteria also cannot identify cryptic species that differ in non-morphological traits

C. Morphological Species Concept



• views a species in terms of its ecological niche applies to sexual and asexual species and emphasizes the role of disruptive selection is widely used for viral species (in addition to genetic homologies)

D. Ecological Species Concept



• defines a species as the smallest group of individuals on a phylogenetic tree (monophyletic group) applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species on phylogenetic trees, an ancestral population plus all of its descendants is called a monophyletic group or clade, which is identified by synapomorphies, homologous traits inherited from a common ancestor that are unique to certain populations or lineages

E. Phylogenetic Species Concept



Phylogenetic Species Concept


• This concept can be applied to any population, but there are disadvantages: phylogenies are currently available for only a tiny (though growing) subset of populations on the tree of life would probably lead to recognition of many more species than either of the other species concepts

Phylogenetic Species Concept


• A key event in the potential origin of a species occurs when a population is somehow severed from other populations of the parent species. With its gene pool isolated, the splinter population can follow its own evolutionary course and become reproductively incompatible• Two modes leading to reproductive barriers can be distinguished allopatric speciation sympatric speciation

Part 3 – Speciation


• Allopatric speciation occurs when geographic isolation creates a reproductive barrier (extrinsic mechanisms)• Sympatric speciation occurs when a reproductive barrier is created by something other than geographic isolation (intrinsic mechanisms)



• Genetic isolation happens routinely when populations become physically separated. Physical isolation, in turn, occurs in one of two ways: dispersal or vicariance. dispersal occurs when a population moves to a new habitat, colonizes it, and forms a new population vicariance occurs when a physical barrier splits a widespread population into subgroups that are physically isolated from each other• Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation

A. Allopatric Speciation



Allopatric Speciation by Dispersal or Vicariance


• Geographic separation prevents species from mating• Speciation occurs only with the evolution of reproductive barriers between the isolated population and its parent population

Allopatric Speciation


• The definition of barrier depends on the ability of a population to disperse for example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen• Separate populations may evolve independently through mutation, natural selection, and genetic drift for example, speciation of snapping shrimp (Alpheus) populations due to separation by the Isthmus of Panama



A. harrisii A. leucurus

Physical Isolation and Reproductive Barriers


Physical Isolation and Reproductive Barriers


• Regions with many geographic barriers typically have more species than do regions with fewer barriers• Reproductive isolation between populations generally increases as the distance between them increases however, barriers to reproduction are intrinsic; separation itself is not a biological barrier



EXPERIMENT

Initial populationof fruit flies(Drosophila

pseudoobscura)

Some flies raisedon starch medium

Mating experimentsafter 40 generations

Some flies raised onmaltose medium

Allopatric Populations and Reproductive Isolation


RESULTS

Female

Starch Maltose

Mal

eM

alto

seSt

arch

Number of matingsin experimental group

22 9

8 20

FemaleStarch

population 1

Mal

eSt

arch

popu

latio

n 2

Number of matingsin control group

18 15

12 15

Starchpopulation 2

Star

chpo

pula

tion

1

Allopatric Populations and Reproductive Isolation


B. Sympatric Speciation


• In sympatric speciation, speciation takes place in geographically overlapping populations• can occur if a genetic change produces a reproductive barrier between mutants and the parent population• may be the result of: polyploidy extreme habitat differentiation sexual selection


Sympatric Speciation


Polyploidy


• is the presence of extra sets of chromosomes due to accidents during cell division an autopolyploid is an individual with more than two chromosome sets, derived from one species an allopolyploid is a species with multiple sets of chromosomes derived from different species• is much more common in plants than in animals many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids


Autopolyploidy


Allopolyploidy


Extreme Habitat Differentiation


• Sympatric speciation can result from the appearance of new ecological niches for example, populations of the North American maggot fly prefer to live either on native hawthorn trees or on more recently introduced apple trees although they are not yet separate species on the basis of any species concept, apple flies and hawthorn flies are diverging


Extreme Habitat Differentiation


Sexual Selection


• Sexual selection can drive sympatric speciation such selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria

Normal lightMonochromatic

orange light

P. pundamilia

P. nyererei


Comparison with Allopatric Speciation


• In allopatric speciation, geographic isolation restricts gene flow between populations reproductive isolation may then arise by e. g., natural selection or genetic drift, in the isolated populations• In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species sympatric speciation can result from polyploidy, natural selection, or sexual selection


• differ among organisms and can occur in time scales a slow rate of speciation evidenced by a living horseshoe crab (13 species) and a 300 million year-old fossil a rapid rate of speciation evidenced by Galapagos finches which have diversified into 13 species within the last 100,000 years


C. Rates of Speciation


• Gradual model traditional evolutionary trees diagram the descent of species as gradual divergence


D. Dynamics


• Punctuated equilibrium is a contrasting model of evolution states that species most often diverge in spurts of relatively sudden change accounts for the relative rarity of transitional fossils and hence appears to be a more accurate view of speciation dynamics

Dynamics


Learning Objectives and Check of Understanding

BIOL101 Introduction to Biology B

• Compare and contrast the different species concepts?• Distinguish prezygotic and postzygotic barriers.• Differentiate allopatric and sympatric speciation.• Compare and contrast the models describing the dynamics of speciation.


Reading Assignments

• Campbell: Chapter 24• Sadava: Chapter 23

BIOL101 Introduction to Biology B


Part 1 – A few reminders from lecture 3Part 2 – Classification and TaxonomyPart 3 – Phylogeny or Systematics

Lecture 4 – 07/08/2011Classification and Phylogeny

Brief Outline of the Upcoming Lecture

biol102

Documents

pearson education

species conceptscopyright

biological species conceptpart

independent population

large population

pearson benjamin cummings

hardyweinberg equilibrium

gene flow migration